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Merge done to enable documentation be be fixed, because of removal of
alias directive and addition of related algorithms directive.

Signed-off-by: Karl Palmen <karl.palmen@stfc.ac.uk>

* Rename HDFDescriptor to NexusDescriptor
* Rename {HDF,NonHDF} -> {Nexus,Generic} in FileLoaderRegistry
* Rename DECLARE_HDF_FILELOADER_ALGORITHM ->
  DECLARE_NEXUS_FILELOADER_ALGORITHM

Refs #7461

Refs #7263

The checks for the headers at the start of HDF4 & HDF5 files have been
put in a static isHDF member to encapsulate the details about
checking for a HDF file and make it reusable.
It will not retain an open file handle to the file so does not
inherit from the standard FileDescriptor.
Refs #7263